Valves having parts made of resin are advantageous in that they can be mass-produced by injection molding, are resistant to corrosion, and inexpensive to construct.
One attempt to provide an increased mechanical strength to the body of such a valve has been to increase the thickness of the molded valve parts. However, thick valve parts cannot easily be molded, and their mechanical strength has proven unsatisfactory. When assembling valves, it is necessary to fasten seal rings, gaskets and other parts under a prescribed load. Prior resin-molded valves include a holder ring for reinforcing and holding a resin housing, and a reinforcement metal cover for protecting the resin housing against external shocks and internal fastening stresses. The valves may additionally have a valve cover for pressing and holding an inner valve body or a seal ring, the valve cover being fastened in position by a plurality of fasteners (mostly of metal) such as fastening bolts.
The prior resin-molded valves, therefore, require separate members made of metal in most cases for increasing the mechanical strength of the valve parts and fastening them under a prescribed load. These additional components have been an obstacle to efforts to achieve a reduction in the production cost and a full degree of corrosion resistance.